This invention relates to a cooling system for an infrared detector, and in particular, the cooling system that uses a linear motor to operate a compressor.
Infrared (I/R) detectors are used for detection of infrared energy and it is usually necessary to maintain the detector at a cryogenic temperature. An example of a prior art cooling system is disclosed in U.S. Pat. No. 3,851,173 entitled "A Thermal Energy Receiver". In the case where the infrared detector is part of a night sight, the integration of linear resonant coolers into the infrared night sight has encountered several problems. A large external magnetic field is generally emitted by the compressor motor which causes EMI problems in the infrared system. The slightest relative motion between the magnetic field and the conductors within the field induces electrical noise on the video circuit which is connected to the infrared detectors causing a degradation in the video output.
Another problem is the vibration caused by the compressor at its resonant frequency. In infrared systems, precision optics are used that are sensitive to vibrations. This, of course, also causes also causes degradation in the video output.
Several schemes have been employed to minimize the effect of EMI interferences or noise from the linear compressor. Heavy shielding of the compressor to intercept the external magnetic field, extensive shielding of the video circuit, and placing the compressor more remotely from the electronics have been tried. Each of these solutions have undesirable consequences.
One solution for reducing vibration has been to include a spring mass type harmonic balancer within the compressor. However, with a balancer any variation in the frequency of operation results in a decrease in the balancer efficiency. Also, external sources of vibration can excite undamped oscillations.
An alternate method of a compressor balancing has been suggested in a NASA patent application Ser. No. N85-21404 in which a pair of mutually opposed pistons moving in unison produces a compression wave without transmitting reaction forces to the compressor or structure. This method achieves satisfactory balancing, however, the required technology development for the device as described has not been demonstrated.